This invention relates to self-oscillating mixer circuits for converting an R-F (radio frequency) signal to an IF (intermediate frequency) signal, particularly but not exclusively self-oscillating mixer circuits suitable for use as the front end of a frequency-modulated continuous-wave (FMCW) short-range radar. The invention also provides a simple frequency-modulated radar comprising such a self-oscillating mixer circuit, for example an inexpensive short-range FMCW radar suitable for use in cars or other vehicles for collision avoidance purposes or speedometer applications.
Self-oscillating mixer circuits for converting an RF signal to an IF signal are known, comprising an RF oscillator having an RF line, a common terminal and an IF line; the RF line serves for transmitting RF signals to and from the oscillator, and the IF line serves for deriving from the oscillator an IF signal which results from the mixing of the RF signals in the oscillator. U.S. Pat. No. 4 219 779 describes one such known self-oscillating mixer circuit in which the RF oscillator is formed by a field-effect transistor connected in a feed-back path. In this known circuit the IF line is connected to the drain of the transistor; the RF line is connected to either the gate or the source, depending on whether a common source configuration or common gate configuration is used in the circuit.
The whole contents of U.S. Pat. No. 4 219 779 are hereby incorporated herein as reference material. In accordance with the teaching in U.S. Pat. No. 4 219 779, a dielectric resonator is coupled to the RF line in order to improve the stability of the oscillator frequency and to suppress leakage of a local oscillator frequency into the RF line input.